This invention relates to a process and device for the anaerobic decomposition or degradation of a waste stream containing organic substrate in which the organic substrate being treated is conducted in a downflow mode through a filter bed arranged in a treatment zone, i.e., a reactor. The filter bed serves as a carrier for the settling of microorganisms therein or thereon, and with the substrate which has been treated, at least in part, in the filter bed, being recycled from the bottom or lower region to the top of the treatment zone for being passed therethrough again for further treatment or degradation.
In typical processes of this type wherein anaerobic microbal transformation, i.e., degradation, of organic substrate primarily into methane and carbon dioxide is effected, there is presented a significant problem since, in the case of complex substrate materials, i.e., a portion of the organic load, expressed as COD, is present in a form that is difficult to metabolize, e.g., as undissolved solids, polymers or polycondensed aromatics. Wastes of this kind are obtained e.g. from the food industry or the chemical industry with the complex substrates being e.g. fats, proteins, carbohydrates, phthalic acids etc.
These substances must first be decomposed into smaller parts or substances that can then be attacked by fermentative bacteria. As a result of this two stage reaction, long reaction times and large-volume reactors are required. Therefore, in order to reduce reaction times, anaerobic high-efficiency reactors are used which, as a rule, are designed as upflow sludge bed reactors, or as downflow filter bed reactors. In both types of reactors, large volumes are required to achieve adequate decomposition performances which can be achieved only by a high degree of retention of the only slowly reproducing anaerobic biomass therein.
A recent summary of anaerobic processes and anaerobic reactors presently in use is given by R. E. Speece in Environm. Sci. Technol. Vol. 17. No. 9 (1983), pages 416 to 427.
In the case of the use of an upflow type sludge reactor, there is the danger, especially in the case when there occurs a high hydraulic load, and/or with high specific generation of gas, that the efficiency of the reactor will be reduced by sludge decanting from the top through the discharge thereof, which results in a loss of active biomass and consequently reduced efficiency. Investigations with distillery waste showed, that increased sludge losses occurred with specific gas production rates above 4 m.sup.3 /m.sup.3 of reactor volume. Furthermore, achievement of a highly concentrated sludge bed depends on the availability of granular sludge settling which settles well, the formation of which cannot be controlled in this type of reactor.
In the case a downflow filter bed reactor is used, a substantial portion of the biomass is firmly fixed to surfaces of support elements therein, or, in the case support elements having an open cell structure are used, the biomass is fixed in the interior of the support elements. In order to obtain a thorough mixing, to avoid clogging, and/or load equalizing in the case of peak loads, a partial stream which is a multiple of the feed stream is recycled from the bottom to the top of the filter bed. As contrasted with an upflow type reactor, loading in downflow operation has the advantage that no process impairing foaming occurs. However, a disadvantage is presented in that a substantial portion of the unfixed, i.e., suspended, biomass therein will be flushed out from the reactor with the purified effluent. Thus, the decomposition performance of the reactor is hampered. Furthermore, there results a relatively increased fatty acid content in the effluent, as compared to an upflow type reactor, that results in annoying odors since it is not possible to recycle the entire quantity of the feed stream and thus, a portion of the feed remains in the reactor for only a relatively short time which is insufficient to effect full or complete degradation.